1. Field of the Invention
The present invention relates to a photo sensor of a photo type touch panel, and more particularly, to a photo sensor of a photo type touch panel with increased working area.
2. Description of the Prior Art
Please refer to FIG. 1 and FIG. 2 together. FIG. 1 is a diagram showing a photo type touch panel 100 of the prior art. FIG. 2 is a diagram showing a photo sensor of the photo type touch panel in FIG. 1. As shown in the figures, the photo type touch panel 100 comprises a plurality of pixels P arranged in a matrix form, a plurality of data lines D for transmitting display data to the plurality of pixels P, a plurality of scan lines W for transmitting scan signals to the plurality of pixels P, a plurality of photo sensors 110, a reading unit 120, and a plurality of scan lines G for controlling the photo sensors 110. Each photo sensor 110 comprises a transistor 112, a capacitor 114, and a photo transistor 116. A control terminal of the transistor 112 is electrically connected to a first scan line G1. The capacitor 114 is electrically connected to a first terminal of the transistor 112. The reading unit 120 is electrically connected to a second terminal of the transistor 112 for reading a voltage level of the capacitor 114 when the transistor 112 is turned on. A control terminal of the photo transistor 116 is electrically connected to a second scan line G2, a first terminal of the photo transistor 116 is electrically connected to the capacitor 114, and a second terminal of the photo transistor 116 is electrically connected to a third scan line G3.
Please refer to FIG. 3, and refer to FIG. 2 as well. FIG. 3 is a diagram showing related signals of the photo sensor of the prior art. When the photo sensor 110 operates, in a first period T1, a voltage signal VG1 of the first scan line G1 is risen to high potential for turning on the transistor 112, a voltage signal VG2 of the second scan line G2 is at low potential (−aV) for turning off the photo transistor 116, and a voltage signal VG3 of the third scan line G3 is also at low potential (−bV), wherein a voltage difference between the voltage signal VG2 and the voltage signal VG3 is kept at a specific value. The reading unit 120 then reads the voltage level of the capacitor 114 and resets the voltage level of the capacitor 114 to a reference level.
In a second period T2, the voltage signal VG1 of the first scan line G1 is lowered to low potential for turning off the transistor 112, the voltage signal VG2 of the second scan line G2 is risen to high potential (−aV+ΔV1) for turning on the photo transistor 116, and the voltage signal VG3 of the third scan line G3 is also risen to high potential (−bV+ΔV2) for keeping a specific voltage relationship with the voltage signal VG2, and the third scan line G3 charges the capacitor 114 for making the voltage level of the capacitor 114 equal to the voltage level of the third scan line G3.
In a third period T3, the voltage signal VG1 of the first scan line G1 is kept at low potential for turning off the transistor 112, the voltage signal VG2 of the second scan line G2 is lowered to low potential (−aV) for turning off the photo transistor 116, and the voltage signal VG3 of the third scan line G3 is also lowered to low potential (−bV) for keeping the specific voltage difference with the voltage signal VG2. Although the photo transistor 116 is turned off, when the photo transistor 116 is illuminated, a leakage current flowing through the photo transistor 116 is larger according to a voltage difference between the control terminal g (gate terminal) and the second terminal s (source terminal); and when the photo transistor 116 is not illuminated, the leakage current flowing through the photo transistor 116 is smaller. Therefore, the voltage level of the capacitor 114 is gradually decreased due to the leakage current of the photo transistor 116.
In a fourth period T4, the voltage signal VG1 of the first scan line G1 is risen to high potential for turning on the transistor 112, the voltage signal VG2 of the second scan line G2 is kept at low potential (−aV) for turning off the photo transistor 116, and the voltage signal VG3 of the third scan line G3 is also kept at low potential (−bV) for keeping the specific voltage difference with the voltage signal VG2. The reading unit 120 then again reads the voltage level of the capacitor 114 for determining whether the photo sensor 110 is touched according to the voltage level of the capacitor 114.
Please refer to FIG. 4. FIG. 4 is a diagram showing characteristic curves of the photo sensor 110 of the prior art before and after long term operation. A horizontal axis of FIG. 4 represents the voltage difference Vgs between the control terminal g and the second terminal s of the photo transistor, and the vertical axis of FIG. 4 represents the voltage level Vout read by the reading unit 120. The reading unit determines how much electricity leaked from the capacitor 114 according to how much electricity charged to the capacitor 114, thus if the voltage level Vout read by the reading unit 120 is higher, the leakage current of the capacitor 114 is larger. The voltage level Vout read by the reading unit 120 can be utilized to determine whether the photo sensor 110 is touched. For example, when the voltage level Vout read by the reading unit 120 is higher than a predetermined value, it is determined that the photo sensor 110 is touched; and when the voltage level Vout read by the reading unit 120 is lower than the predetermined value, it is determined that the photo sensor 110 is not touched. A working area of the photo sensor 110 may significantly changed when the control terminal g and the second terminal s of the photo transistor 116 are continuously applied with working voltages for a long time or the photo transistor 116 is illuminated for a long time. As shown in FIG. 4, at a beginning state (without being applied with working voltages or illuminated for a long time), a working area B formed between a illuminated curve and a non-illuminated curve is located between 0V to −4V of the horizontal axis. After being applied with working voltages or illuminated for a long time, the photo transistor 116 become easy to leak current, so as to change the illuminated curve and the non-illuminated curve, and a working area A is shifted to be located between −3V to −6V of the horizontal axis.
The working area B of the photo sensor 110 at the beginning state is very narrow, and only slightly overlaps with the working area A after long term operation. When the voltage difference Vgs between the control terminal g and the second terminal s of the photo transistor 116 is set to a specific voltage difference (such as −2.5V), the voltage difference Vgs between the control terminal g and the second terminal s of the photo transistor 116 can not be located at the working area A after long term operation. Therefore, after the photo sensor 110 of the photo type touch panel 100 of the prior art operates for a long time, it may cause the reading unit 120 to misjudge.